knock

v.

Old English cnocian (West Saxon cnucian), "to pound, beat; knock (on a door)," likely of imitative origin. Meaning "deprecate, put down" is from 1892. Related: Knocked; knocking. Knock-kneed first attested 1774. Knock-down, drag-out is from 1827. Command knock it off "stop it" is first recorded 1880, perhaps from auctioneer's term for "dispose of quickly:"

At the commencement of the sales, he gave every one that wanted to purchase a paper containing a description of the lands that were to be sold; and, as the sales were cried, he called over the numbers and described the land; and when it got up to one dollar and a quarter an acre, if no body bid, after it was cried two or three times, he would say, knock it off, knock it off. [U.S. Senate record, 1834]

knockoff

n.

"cheap imitation," 1966, from the verbal phrase knock off "do hastily;" in reference to the casual way the things are made.

knock off

Also,knock out. Dispose of or produce easily or hastily, finish, as in A writer of detective novels, he knocks off a book a year, or We can knock out a rough drawing in a few minutes. The first colloquial usage dates from the early 1800s, the variant from the mid-1800s.

3.

Get rid of, reduce, as in She knocked off twelve pounds in a month, or They knocked off one-third of the original price.
[ ; early 1800s
]

4.

Kill, murder, as in They decided to knock off the old lady.
[ ; early 1900s
]